This invention relates to telephone communication equipment and, more particularly, to a compact configuration of circuitry employed in separation of video/data signals and voice signals in the central office of a telephone company (telco) and at the customer premises.
Telephonic communication among homes, businesses, and other facilities is accomplished in well-known fashion via one or more central offices of a telephone company with the electrical signals being communicated via pairs (twisted pairs) of electrically conducting wires. Voice signals are transmitted in a frequency band of a few kilohertz (kHz), typically 0-4 kdz, Wherein the twisted wire pair can carry signals ranging from DC (direct current) to the high frequency cutoff of the transmission circuitry. For normal voice signal transmission the high frequency cutoff is in the range of approximately 3-4 kHz. However the twisted wire pair is capable of transmission of electrical signals of higher frequency suitable for computer modem digital communication, and for compressed video signals such as video signals transmitted in the MPEG-2 format. Unlike transmission of such signals by coaxial cable and by satellites wherein the signals are modulated onto carriers, in the transmission of the signals via the twisted wire pair there is no use of a carrier and the signals are transmitted in their baseband format. As a result, there is considerable interest in the use of telephone lines of the telephone company (telco) for communication of high speed modem signals and compressed video signals in addition to voice signals among the homes, businesses, and other facilities who are the telephone subscribers.
In the communication, via the telephone lines, of the combined signals of voice plus video, or other higher frequency signals such as the high speed data transmitted by modem signals, the fidelity of the voice signal is enhanced by separating the voice signal from the high frequency signals. The high frequency of signals which carry video and/or data may be referred to hereinafter as the video signals for convenience of description. Voice circuitry, such as transmitter and receiver, is employed for handling the voice communication, and RADSL (rate adaptive digital subscriber line) circuitry is employed for handling the video and the data. At the telco and at the subscriber, the voice and the video are handled separately by the voice and RADSL circuits, while between telco and the subscriber, both voice and video are communicated by a common twisted pair.
In the telco, maintenance of the numerous racks of cards presents a problem in that there is a need for bypassing the voice channels of cards which are being extracted for service so as to enable a continuation of service even during the absence of the card. While such bypassing can be accomplished by a manual switching of circuitry, a manual procedure represents an inconvenience and introduces complications to the maintenance process. Furthermore, it is desirable to construct the circuits with a configuration amenable to a compact packaging of the filters of numerous circuits upon a single card, thereby reducing the storage space and increasing accessibility of the circuitry to maintenance personnel.
The aforementioned problems are overcome and other advantages are provided, in accordance with the invention, by a construction individual ones of filter circuits, an arrangement of plural filter circuit on a single card, and interconnection and mounting of numerous cards on a single rack to facilitate maintenance.
In a communication system employing the invention, a low pass filter is employed at the telco for each telephone communication channel, and another low pass filter is employed at the subscribe terminal. In each case the low pass filter serves for separation of the video from the voice. Each of the filters employs a common form of construction of inductors and capacitors, arranged in a balanced line configuration (inductors in both legs of the ladder with capacitive cross branches), wherein, in accordance with a feature of the invention, each of the inductors is provided with a toroidal core. Thereby, the inductor provides a desired amount of inductance while presenting a lower insertion loss to the circuit. The lower insertion loss, in turn, allows for increased distance between subscriber and telco. In accordance with a further feature of the invention, the size and material composition of the toroidal core of an individual one of the inductors are selected to prevent the inductor from being driven into saturation. In the construction of the low pass filter, plural inductors are located in a common plane, such as on a printed circuit board, with axes of the toroids being parallel. This inhibits magnetic coupling between the inductors. Capacitors of the filter are located alongside the inductors to facilitate manufacture and packaging.
At the telco, eight filters for eight subscriber channels are constructed on a single Eurocard, which may be referred to hereinafter as a card for convenience. Fifteen cards are mounted on a rack and connect via a set of connectors through a back plane of a cabinet to circuitry of the central office and the subscribers. Surge protection for each communication channel is provided by a sidactor, also located on the card. On each card, a common mode choke, generally employed in telephone circuits, is connected between the surge protector and its corresponding filter. During normal operation, all communications between the central office switching equipment and an individual subscriber pass through a respective filter located on one of the cards.
In accordance with the invention, a switching connector is also provided on the back plane for each of the cards. The switching connector comprises a set of spring tangs, each of which serves as a bypass in a respective one of the telephone communication channels for communication of voice, but not video, between the subscriber and the central office switching equipment upon removal of the card for service. The system of switching connectors permits servicing of individual cards without requiring removal of an entire shelf or rack of cards.
In each of the low pass filters, inductors of differing values and capacitors of differing values are to provide the desired filter characteristics. In the case of cards mounted in the rack at the central office, it is convenient to place all components, including the filter components, the chokes, and the surge protectors on one surface of the substrate of the card. In a similar fashion, a low pass filter is employed in a subscriber station in series with a telephone for noise reduction.
It is noted also that, the features of construction of the filters are useful also for connection of the telephone channel, the twisted pair of wires, at a subscriber terminal. With respect to emplacement of a low pass filter at the entry point of the twisted pair at a subscriber home or other facility, it is noted that a suitable electrical configuration of the low pass filter is the same as that described above for the filters located on the card. However, in the case of the card mounted filter at the central office, the filter design is for an impedance of 900 ohms while, for the filter located at the subscriber entry point, the filter design is for 600 ohms. To accomplish this change of impedance, the values of the filter components of the subscriber low pass filter differ from the values of the corresponding components of the central-office low pass filter.
Locating a low pass filter at the subscriber entry point allows a filtered incoming signal to be split among plural telephone circuits of the subscriber. Alternatively, such a filter may be connected to each subscriber telephone, in which case the power splitting filter at the entry point is not needed. The subscriber filter may be constructed on a substrate using toroid inductors in a manner similar to that described above for the central office filters. However, the construction of the subscriber low pass filter is simpler than the construction of the central office card because the components of only one filter channel are supported by the substrate of the subscriber filter.